3/8 fuel lines + 5/16 sending unit

Well.....not exactly. First it depends on whether you REALLY need 3/8 lines, and if it's that borderline IE that you run so much HP that you absolutely MUST have the full flow of a 3/8 line, then you should probably be running HALF inch lines

The fact is that a very short piece of 5/15 tube in a 3/8 system is not going to add a huge drop in flow rate. This has to do with friction in the hose/ tubing.

Realistically, how much HP are you figuring on?